Please let me know that what is gradient and linear-gradient? Why takes different values here like
2nd line webkit-gradient(linear, left top, left bottom,
3rd line -moz-linear-gradient(top and
4th line background: linear-gradient(-90deg,

gradients are basically a blend of two colours over a progression of space between the two colours. Linear gradients are when two colours that are paralel to each other blend and merge to produce an attractive colour range in one line.

While creating websites, gradients have been introduced in CSS3 which is not supported by all web browsers. For that purpose, webkits,etc are used according to the kit used by each web browser.

In the code you have given above

background: #6a6a6a url(images/nav-bar-bg.png) repeat-x;

is used for every browser regardless as it supports all browsers.

background: -webkit-gradient(linear, left top, left bottom, from(#b9b9b9), to(#6a6a6a)); is used by google chrome and safari

background: -moz-linear-gradient
is used by Mozilla Firefox

background: linear-gradient(-90deg, #b9b9b9, #6a6a6a);
is used by internet explorer 9